Why Do Some Pestos Contain Ascorbic Acid?

Short answer
Ascorbic acid is another name for vitamin C, a natural antioxidant. Pesto makers add it to their sauces in tiny quantities to help keep the sauce looking fresh and vibrant over its long shelf life.

Long answer
Plenty of ingredients that producers put into food might at first sound scary. Ascorbic acid joins titanium dioxide, methylcellulose, potassium sorbate, and dozens of other ingredients used by sauce makers to improve their products' taste, vibrancy, and shelf life.

Ascorbic acid is simply the scientific name for vitamin C. While nothing completely stops herbs like basil from dulling over time, ascorbic acid has natural antioxidant properties. It is a way manufacturers can help slow down the browning process.

The acid is normally added in powdered form, but only in tiny quantities. It rarely makes up more than 0.005% of the sauce's total weight.

The official pesto recipe does not include ascorbic acid because it slows the speed of oxidation, which is unnecessary if you plan to eat the sauce the same day. If you plan to keep your sauce for several days, the simplest thing to do is blanch your basil in boiling water for a few seconds. That will kill the enzymes responsible for browning and keep the sauce looking vibrant green for longer.